Well, it's not the seeds, because seedless grapes have also caused liver failure in dogs.
Like I said in the post above...this is in dogs, and some have said that grapes have caused glider deaths.
I have looked and have not found any thing that confirms it with solid proof like a necropsy of a glider(s)death due to grapes, but I am not going to risk it, when there so many other safe fruit options for them, so I just skip them.